The invention relates to wide pass-band microwave down converters in which the intermediate frequency is low with respect to the frequency of the carrier.
It is known that down converters of this type must be balanced to reduce the effect of the noise of the local oscillator.
It is also known that the decoupling of the three waves present in the down converter, in relation to each other, raises difficulties when the pass-band is wide and when the intermediate frequency is low.
Two solutions to the problem of decoupling the three waves have been proposed in U.S. Pat. Nos. 3,584,306 filed on Sept. 18, 1967 and 3,638,126 filed on Jan. 28, 1972. The synthesis of the basic arrangements disclosed in these two patents is shown in FIG. 1 of the catalogue of SPACE KOM Company. This figure has been reproduced in the present application as prior art. In this down converter, the waveguide 1 is supplied by the local oscillator, the microwave is introduced into cavity 2 constituted by the end of a waveguide terminated by a short-circuit, and the intermediate frequency wave isolated by filter 3 is available at coaxial connector 4. The local wave is introduced into cavity 2 by means of a conductor passing through the waveguide 1 and extending by a flat conductor 5 located in the horizontal median plane of cavity 2.
However, the configuration of the electric field generated by the conductor 5 is symmetrical with respect to said median plane. The cavity 2 can be excited by the local wave because of the presence of two diodes 6 serially connected in the same direction, each diode having an electrode in contact with conductor 5 and an electrode coupled to the waveguide in order to form a dissymmetry between the two components of the field produced by the conductor 5. The beats between the local wave and the microwave appear on conductor 5 because of the non-linearity of diodes 6, and the lower frequency sideband is isolated so as to form the intermediate frequency wave. The structure shown in FIG. 1 is only compatible with diodes mounted in a housing but the latter limits the frequency at which the diodes are used.